The invention relates to a servo valve arrangement with a high-pressure, low-pressure and motor connection and with a pressure control valve arranged between the high-pressure connection and motor connection and a return control valve arranged between the motor connection and low-pressure connection. The pressure control valve normally is closed and the return control valve normally is open. Both valves have axially displaceable control parts which are arranged equiaxially with respect to one another and can be actuated by a common actuating member, in particular a ram, while closing the return control valve and opening the pressure control valve.
In the case of conventional hydraulic power steering systems currently used in motor vehicles, a hydraulic medium flows virtually constantly through the associated servo valve arrangement. This is true specifically even when no servo force is required at all such as, for example, in normal straight-ahead travel. The servo valve arrangement consists in principle of two parallel throttle routes which are arranged between a pressure connection and a low-pressure connection. Each comprise two throttles which are controlled in opposite directions, are arranged in series, and between which a motor connection branches off in each case. Depending on the direction and magnitude of an actuating moment to be applied at the steering wheel, the input-side throttle of the one throttle route is increasingly opened and, at the same time, the output throttle is increasingly throttled, while the throttles of the other throttle route are controlled in the opposite direction in each case. In this manner, a pressure difference which can be controlled according to direction and magnitude is generated between the motor connections of the two throttle routes, such that an appropriate servomotor generates a controllable servo force in one direction or the other. For operating states in which no servo force is required, all the throttles assume a central position 80 that the medium flows evenly through both throttle routes and there is pressure equilibrium between the motor connections.
Since, in these known power steering systems, a constant hydraulic flow must be maintained, power is required constantly.
In terms of a reduction in the power requirement of motor vehicles, it is basically known to provide power steering systems with a so-called "closed center", i.e. as long as no servo force is required, the servomotor has no connection at all to the pressure source. In contrast, in this state the servomotor is only connected to the low pressure connection or hydraulic reservoir in order to allow passive mobility of the servomotor.
In this context, attempts have already been made to implement power steering systems using the servo valve arrangements specified above. Essentially, two servo valve arrangements of the type specified above then have to be arranged parallel to one another between a pressure source or a pressure accumulator and a hydraulic reservoir. Depending on the actuating moment to be applied at the steering wheel, the pressure control valve of one servo valve arrangement or the other is then opened to a greater or lesser extent. This is so that in each case the motor connection of the one servo valve arrangement or the other is connected with a controllable throttle resistance to the pressure source or the pressure accumulator and, at the same time, is blocked off relative to the low-pressure connection. The servomotor connected between the motor connections of the two servo valve arrangements thus generates a servo force in one direction or the other.
There is therefore needed a servo valve arrangement which guarantees particularly good control characteristics.
These needs are met according to the present invention by designing the pressure control valve as a seat valve.
In this manner, it is guaranteed that the closed state of the pressure control valve is combined with a precisely reproducible position of the associated control part, and any adjustment of the control part immediately entails a controlled opening of the pressure control valve. Seat-controlled valves are otherwise distinguished by the fact that they close virtually without leakage.
According to a first embodiment of the present invention, the control parts of the pressure and return control valve can be arranged as parts of a common valve piston. In this manner, the return control valve only allows a throttled connection between the motor connection and the low-pressure connection when the pressure control valve is closed. It is thus guaranteed, on the one hand, that the return control valve closes quickly when the pressure control valve opens. On the other hand, the return control valve can act in the open state in the sense of damping movements of the servomotor.
According to a particularly preferred alternative embodiment of the invention, the control part of the return control valve is made to be displaceable to a limited extent relative to the control part of the pressure control valve and to be coupled thereto in terms of drive. This is done in such a manner that the pressure control valve only opens after the return control valve has closed. In this way, for virtually all operating states, it can be guaranteed that the pressure connection and low-pressure connection remain separated from one another and that virtually no hydraulic power losses whatsoever occur.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.